Modeling of Collision-Induced Excitation and Quenching of Atomic Nitrogen

TL;DR

This study models collision-induced excitation and quenching of atomic nitrogen, providing detailed cross sections and rate coefficients relevant for plasma formation in hypersonic shock-waves, with improved methods for considering spin-orbit effects.

Contribution

The paper introduces a new scaling method for spin-orbit coupling in collision calculations and provides comprehensive cross sections for nitrogen atom collisions at high energies.

Findings

Spin-orbit and derivative couplings significantly influence excitation and quenching.

Derivative coupling effects are generally stronger than spin-orbit effects.

Identification of two pathways for N(2D) relaxation, including an Auger-like mechanism.

Abstract

Excited atomic nitrogen atoms play an important role in plasma formation in hypersonic shock-waves, as happens during spacecraft reentry and other high velocity vehicle applications. In this study, we have thoroughly studied collision induced excitation (CIE) associated with two colliding nitrogen atoms in the N(4S), N(2D) and N(2P) states at collisions energies up to 6 eV, using time-independent scattering calculations to determine cross sections and temperature-dependent rate coefficients. The calculations are based on potential curves and couplings determined in earlier MRCI calculations with large basis sets, and the results are in good agreement with experiment where comparisons are possible. To properly consider the spin-orbit coupling matrix, we have developed a scaling method for treating transitions between different fine-structure components with calculations that only require calculations with two coupled states, and with this we define accurate degeneracy factors for determining cross sections and rate coefficients that include all states. The results indicate that both spin-orbit and derivative coupling effects can play important roles in collisional excitation and quenching, and that although derivative coupling is always much stronger than spin-orbit, there are many transitions where only spin-orbit can contribute. As part of this, we identify two distinct pathways associated with N(2D) relaxation, including one Auger-like mechanism leading to 2N(2D) that could be important at high temperature.